In rolling stands for strip or sheet it is known that a desired deformation has to be made on the working rolls in order to contrast the elastic deflection caused thereon by the rolled stock passing through.
For it is well-known that the behaviour of the working rolls during the rolling process can be likened to a beam constrained at the ends, in such a way that the rolling pressures generate on the rolls a deflection which is greatest in correspondence with the centre and which is a function of the width of the rolled stock.
If this deflection is not compensated, it causes a disuniformity in the thickness and problems with the planarity of the product, both lengthwise and especially widthwise; this situation is more and more unacceptable; given the high standards of quality required by the market.
In order to limit the elastic deflection of the rolls, and still allow working rolls of a small diameter to be used in order to exploit their intrinsic advantages, four-high rolling stands have been proposed wherein each of the two working rolls is associated with a back-up roll.
The back-up rolls transmit the rolling pressure to the relative working rolls and cause a structural strengthening thereof which limits the deformation.
In this type of stand, the deformation which contrasts the deflection of the working rolls is performed on the back-up rolls which, through contact, transmit the deformation to the working rolls themselves.
The different pressures imparted to the working rolls, depending for example on the different width or the type of product which is to be rolled, cause different deformations of the working rolls and therefore it is impossible to mechanically set a curve to the back-up rolls which is optimum for a range of products and for the whole duration of a rolling cycle.
To solve these problems, a plurality of devices of various types have been proposed--mechanical, hydraulic, pneumatic or other--which are able to vary in line, and possibly in a differentiated manner along the width, the surface profile of the back-up rolls according to the requirements of processing.
However, these solutions only give a limited accuracy and are shown to be inadequate in the case of rolled products which are particularly thin and/or with particularly strict parameters of quality.
Moreover, when it is necessary to correct the configuration of the profile of the back-up rolls during the rolling cycle, it takes a long time to carry out this correction, and therefore it causes a long section of rolled stock to be produced which is unacceptable in quality.
SU-A-1039598 describes a rolling roll comprising means to adjust the profile consisting of electric windings and magnetostrictive converters, that is to say, made of material suitable to be deformed if it is subjected to the action of a variable magnetic field.
The roll comprises a rotation shaft on which the electric windings are attached and a sleeve within the body of which the magnetostrictive converters are located, arranged in concentric rings around the rotation shaft.
This reciprocal configuration of the electric windings and the magnetostrictive rings has the disadvantage that the density of the magnetic field generated is not uniform over all the sections of the magnetostrictive ring and moreover the lines of force are arranged on the ring in an incorrect way.
Moreover, because of the dispersion of the lines of force there is a deterioration, apart from a lack of uniformity, in the effect of mechanical deformation of the magnetostrictive rings.
These disadvantages are even more serious if we consider that it is necessary to control the profile of the rolls with absolute precision, given that an imprecise control has serious consequences on the planarity and uniformity of the surface of the rolled stock.
The present applicant has designed, tested and embodied this invention to overcome the shortcomings of the state of the art and to achieve further advantages.